The number of credit hours
one is taking.
Difficulty also plays a role, though, practically speaking - lab
classes, although they may be only 1, or no, hours, often are more difficult
by far than the accompanying course.
This semester, I had a lab class that took only 2 hours a week, but consumed more of my time than all my other classes combined. The fact that I was learning Java, an entirely new programming language to me, was a big part of that.